Using hydrophobically associative polymers (HAP) to improve the sweep efficiency has been proposed as soon as the early 1980s (U.S. Pat. Nos. 4,529,523, 4,432,881, 4,814,096, WO85/03510, U.S. Pat. Nos. 4,702,319, 4,709,759, 4,638,865, 4,780,517, 4,852,652, 4,861,499, 4,476,169).
HAPs are polymers whose skeleton is hydrophilic, but which comprise along the chains small amounts (some percents by mole) of hydrophobic monomers likely to combine in water in form of hydrophobic nanodomains, also referred to as <<hydrophobic links>>. The latter act as temporary reticulation points and they confer very interesting properties on HAPs, notably as regards their rheology. Their viscosities in concentrated regime are also significantly higher with low shear gradients than those of equivalent non-associative polymer solutions at the same concentration. This major property allows to consider much more favourable (i.e. lower) mobility ratios than with conventional polymers at the same concentrations, and thus to provide a higher sweep efficiency. Furthermore, since the reticulation points between the HAP chains involve low energies, they can be reversibly broken under the effect of shearing. The HAP solutions thus have a clearly more marked shear-thinning character than equivalent non-associative polymer solutions, which favours their injectivity in wells. Finally, in general terms, high viscosities with low gradients are obtained with markedly shorter HAP chains than the conventional polymer chains commonly used in tertiary recovery. The HAP solutions therefore intrinsically have a higher mechanical degradation strength than conventional polymer solutions.
Several experimental studies have been carried out to evaluate the performances of HAPs for EOR applications (Taylor, K. C., Nasr-el-Din, H. A.: “Water-soluble hydrophobically associating polymers for improved oil recovery” Journal of Physical Chemistry B, 19, 265-280 (1998)). However, a specific feature of the use of HAPs for this type of application is the <<abnormal>> behaviour thereof in terms of adsorption on surfaces: unlike conventional polymers, the HAP adsorption isotherms do not tend towards a plateau when the polymer concentration increases (Volpert, E., Seib, J., Candau, F., Green, N., Argillier, J. F., Bai, B., Audibert, A.: “Adsorption of Hydrophobically Associating Polyacrylamides on Clay” Langmuir, 14, 1870-1879 (1998)). This abnormally high adsorption has proved beneficial for well treatment applications of water inflow prevention and profile control type (U.S. Pat. Nos. 4,476,169, 6,364,016).
According to studies on HAP adsorption (Volpert, E., Selb, J., Candau, F., Green, N., Argillier, J. F., Bai, B., Audibert, A.: “Adsorption of Hydrophobically Associating Polyacrylamides on Clay”. Langmuir, 14, 1870-1879 (1998)), their particular adsorption on surfaces in relation to that of non-associative polymers results from the formation of hydrophobic links between the successive adsorbed chain layers. Thus, a first adsorbed HAP layer forms, as in the case of non-associative polymers, i.e. notably by means of Van der Waals type bonds with the surface. Additional HAP chains then bind with those of the first layer without being in contact with the surface. They form hydrophobic links with the hydrophobic groups of the already adsorbed chains. HAP multilayers can thus form, which explains the higher adsorption of HAP in relation to that of the conventional non-associative hydrosoluble polymers. This mechanism is pertinent for static adsorption on surfaces (powders, etc.) as well as adsorption at the surface of pores or pore thresholds in porous media, such as reservoir rocks.
Document U.S. Pat. No. 6,364,016 by Dalrymple et al. describes a method that consists in improving the propagation in depth of polymers intended to reduce the water permeability in the well vicinity by injecting beforehand small molecules that will temporarily occupy the adsorption sites available on the surface of the pores in the immediate vicinity of the well. The polymers injected thereafter will first propagate beyond the zone treated by the small molecules and adsorb further away, where adsorption sites still are available. Then, they progressively replace the small molecules in the well vicinity so as to finalize the water inflow control process. This is a conventional effect of desorption of the small molecules in favour of the near-irreversible adsorption of the polymers of higher molar mass. This method cannot be suitable for EOR applications by HAP injection. In fact, in EOR, the polymers have to be injected over long periods of time and propagate on the reservoir scale so as to sweep the hydrocarbons, which is incompatible with desorption in favour of another adsorption.
In EOR, a strong adsorption is not acceptable. It can in fact first severely penalize the injectivity and depth propagation of the HAPs. Furthermore, it is unfavourable from an economic point of view since the material adsorbed no longer contributes to the viscosity of the aqueous phase, which increases the mobility ratio.
A method allowing to minimize HAP adsorption in porous media therefore is clearly of great significance for EOR applications.
According to the invention, it appears that HAP multilayer adsorption can be controlled and avoided by preventing the formation of a first adsorbed layer of HAP.